Physical And Chemical Methods Of Liquid Dispersion

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The techniques that are used in the dispersion of powders in both aqueous and non-aqueous mediaare quite similar. Dispersants aid wetting and stabilization of a suspension and can be ionic (anionicor cationic) or non-ionic. However, concentrations of dispersants must be low. Otherwise, theirproperties are reduced or even inverted (i.e., cause aggregation or flocculation). In dispersing apowder in a non-aqueous medium, it is critical to select an organic solvent that is a good diluent aswell as dispersant, although there are many organic dispersant aids. Aqueous surfactants areprimarily responsible for reducing surface tension, hence aiding in the dispersion of powders inaqueous systems.

The first task in dispersing a dry powder is the wetting of the sample with diluentor dispersant, if needed. Gentle spatulation of the resulting paste prior to addition of diluent furtheraids the dispersion process. Once the diluent is added, a sample of the slurry can be placed on a slidefor a microscopic evaluation of the effectiveness of dispersion. Providing that the sample is welldispersed, the use of a magnetic stir bar is suggested ...
... to keep the sample fully dispersed, so arepresentative sample can be drawn for analysis while stirring is in progress.

Physical Methods of Liquid Dispersion

Spatulation: Use spatula to break up large clumps but not individual particles.

Sonication: Use sound waves to break up aggregates; use either ultrasonic bath or probe.

Dilution: High dilution can aid in dispersion by allowing more space between particles.

Degaussing coil or heat: Can aid in the dispersion of magnetic particles.

Chemical Methods of Liquid Dispersion

Wetting agents:Used to lower surface tension (e.g., hydrophobicity) between diluent (typicallywater) and particle (usually nonionic agent).

Surfactants: “Surface active agents” increase the surface charge of the particles in order to cause them to repel one another so that they remain properly dispersed while in the suspension.Types of surfactants include:

• Anionic:Imparts negative charge
• Cationic: Imparts positive charge
• Amphoteric: Imparts both negative and positive charges
• Nonionic: Wets particle without imparting charge

Stabilization by Surface Charge

• Adjustment of pH for surface ionization. Adjusting the pH so that the surface is charged.
— Amine, hydroxyl, and carboxyl groups all adsorb a hydrogen ion below their isoelectric pH value (obtained via to zeta potential analysis) and result in a positive (+) charge on the group.
— Amine, hydroxyl, and carboxyl groups all lose a hydrogen ion above their isoelectric pH value and result in a negative (-) charge on the group.
— Typically, 2 pH units above or below the isoelectric pH value will result in stabilization.Stabilization occurs when the zeta potential is at least +30 mV.

• Common Ions
— A solution containing a dissolved ion which is the same as one found in the sample's molecular lattice may help disperse that sample by adsorbing and charging the surface of the particles. However, be careful for solvation effects.

• Multiple-charged Ions
— For ionic particles or particles with polar bonds in water, multiply-charged ions that are not part of the crystal lattice, may adsorb to give a surface charged with soluble salts. (Examples: polyphosphates; hexametaphosphate, pyrophosphate, polysilicate ions)
— For nonpolar organic particles in polar organic media, the surface can be charged by adsorbing a neutral ion-pair.Dissociation then occurs with one part of the ion-pair, desorbing and leaving a charged particle. (Example: trimethyldodecylaminehydroxybenzoate dissociates into a quaternary amine(+) and a polar organic acid(-).)

• Surfactant Ions which charge the surface
— Organic powders: These can adsorb the organic ion of a surfactant with the inorganic counter-ion dissolved in solution, allowing the particle to be wetted and charged, thus repelling each other.
— Organic amines adsorb a hydrogen ion to become positively charged when pH is below the pKb.
— Organic acids lose a hydrogen ion to become negatively charged when pH is above the pKa.
— Again, zeta potential of + 30 mV is achieved when pH is 2 units above pKa or 2 units below pKb.

Reference – https://www.beckmancoulter.com/wsrportal/techdocs?docname=B05577AB.pdf